Stable binary crystal structures

The binary oxides and hydroxides of Ga, In and T1 have been much less extensively studied. The Ga system is somewhat similar to the Al system and a diagram summarizing the transformations in the systems is in Fig. 7.13. In general the a- and y-series have the same structure as their Al counterparts. )3-Ga203 is the most stable crystalline modification (mp 1740°) it has a unique crystal structure with the oxide ions in distorted ccp and Ga " in distorted tetrahedral and octahedral sites. The structure appears to owe its stability to these distortions and, because of the lower coordination of half the Ga ", the density is 10% less than for the a-(corundum-type) form. This preference of Ga "... 

For the hexacoordinate starting complex (1) derived from the crystal structure of the binary complex, the most stable conformer (la, Fig. 6) was found to feature the imidazole ring in His 290 rotated 180° compared to that of the crystal structure, and this conformation of His290 was found to be preferred for all stationary points leading up to the FeIV=0 intermediate. Unless otherwise noted, all energies in this study are given relative to la. 

When the a phase, i.e, the primary solid solution, has only a limited range of stability, other intermediate phases are formed. At particular concentrations of the second component a transformation from one crystal structure to another takes place. In a large number of binary systems, e.g. Gu-Au, Cu-Al, Cu-Sn, a transition from the cubic close packed structure of copper to a body centred cubic structure ()3 phase) occurs at a particular concentration. The phase is stable over a particular range of concentration and at higher concentrations is generally converted to the y-phase which has a complex structure, followed by the e and >) phases which are... 

Crystal structure plays a secondary role in catalysis by the Transition Metal Sulfides. As the periodic trends for HDS of the binary sulfides shows the dominant effect is which transition metal is present in the reaction, this transition metal takes on the structure and stoichiometry of the phase which is most stable in the sulfur containing catalytic environment. The unsupported promoted catalyst systems can be grouped into "synergic" pairs of sulfides. Because these pairs are related to the basic periodic trends of the binary Transition Metal Sulfides through average heats of formation. 

From the chemical point of view it seems to be of particular interest that further (metastable) tellurium subhalides can be obtained by hydrothermal syntheses in acid solutions. In the hydrothermal work on the crystal growth of arsenic telluro-iodides a pure binary tellurium subiodide with the composition Tcsl was obtained as well as a ternary phase with only small amounts of arsenic (AsT Ij The composition Tejl was checked by chemical as well as energy-dispersive X-ray analysis. Crystals of the new tellurium subiodide are shown in Fig. 43 a crystal structure analysis failed because the quality of the crystals was not suitable for single crystal investigation. During annealing of the subiodide a stable phase combination a-Tel -H Te was formed which indicated the metastable character of TesL... 

An important and numerous class of intermetallic compounds AM2 are the cubic Laves phases (MgCu2 type, C15). Many representatives with A or M being one of the transition metals forming stable binary hydrides (see in.B.l) absorb considerable amounts of hydrogen up to compositions AM2H7. Hydrogen occupies tetrahedral interstices of the crystal structure, thereby expanding it. 

The presence of a solvent in the crystallization process alleviates these problems, mainly by lowering the activation energy barrier for the rearrangement of the solute molecules into a new crystalline structure. Thus, the solvent has the classical function of a catalyst and will only affect the kinetics but not the thermodynamics of the transformation. This is valid as long as the solvent does not become part of the crystal structure, that is, is not forming a solvate with the solute. With respect to co-crystals this signifies that the existence of a thermodynamically stable binary compound is not a function of the solvent. 

Similarly, there is high mutual solid solubility between U, Np, and Pu when they occur in their complex crystalline forms at low temperatures. Solid miscibility gaps of only a few atomic percent separate a-Np and /J-Np from a-Pu and /3-Pu, although the terminal solid solutions have different crystal structures. The same may be said of the extent of solid solubility between the low-temperature forms in the systems U-Np and U-Pu, provided that the broadly stable intermediate phases that occur are considered to be transitional extensions of the terminal solid solution phases. Fig. 19.S shows schematically that regions of single-phase solid solution exist over virtually the entire compositional region at low temperatures in the ndghboring binary systems U-Np and Np-Pu [31]. ... 

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